fix: uint128 ambiguousness breaking macos unity build (#5386)

src/libxrpl/basics/mulDiv.cpp

@@ -31,8 +31,6 @@ namespace ripple {
 std::optional<std::uint64_t>
 mulDiv(std::uint64_t value, std::uint64_t mul, std::uint64_t div)
 {
-    using namespace boost::multiprecision;
-
     boost::multiprecision::uint128_t result;
     result = multiply(result, value, mul);